Problem: Simplify the following expression and state the condition under which the simplification is valid. $y = \dfrac{k^2 - 25}{k + 5}$
First factor the polynomial in the numerator. The numerator is in the form ${a^2} - {b^2}$ , which is a difference of two squares so we can factor it as $({a} + {b})({a} - {b})$ $ a = k$ $ b = \sqrt{25} = 5$ So we can rewrite the expression as: $y = \dfrac{({k} + {5})({k} {-5})} {k + 5} $ We can divide the numerator and denominator by $(k + 5)$ on condition that $k \neq -5$ Therefore $y = k - 5; k \neq -5$